Lack of proper nutrition is possible but unlikely. I would be concerned about metabolic diseases; kidney problems, diabetes, liver disease, or protein problems. Various gastrointestinal diseases like inflammatory bowel disease or malabsorptive issues can lead to weight. Often the first sign of cancer is weight loss and loss of muscle mass. Have your cat examined by a veterinarian. Initial tests will include blood work and may include imaging (radiographs or ultrasound).